DNA is a microbial product that is sampled by the mammalian immune system. The ability to reproduce the immunostimulatory activity of bacterial DNA with CpG-rich oligonucleotides has led to the elucidation of optimal stimulatory motifs that influence inflammatory activity such as CpG-DNA (Agrawal and Kandimalla, 2002, Trends Mol. Med., 8:114-121; Krieg, 2002, Ann. Rev. Immunol., 20:709-760).
Toll-like receptors (TLRs) are involved in immune system signaling and, depending on the specific TLR, recognize a set of conserved molecular structures (Gordon, 2002, Cell, 111:927-930). TLR9 is a TLR whose signaling is stimulated by CpG-DNA. Drosophila Toll acts indirectly, in that it is activated by invasive microorganisms by the pathogen-induced proteolytic processing of prospätzle to spätzle (Tauszig et al., 2000, Proc. Nat. Acad. Sci. U.S.A., 97:10520-10525), the actual Toll-binding ligand (Weber et al., 2003, Nat. Immunol., 4:794-800). It has been reported that TLR9 is expressed intracellularly (Ahmad-Nejad et al., 2002, Eur. J. Immunol., 32:1958-1968; Takeshita et al., 2001, J. Immunol., 167:3555-3558), yet it remains unknown in which subcellular compartment TLR9 is expressed, and in which cellular compartment initiation of TLR9 signaling by CpG-DNA occurs.